From:         Steve Noble

Peter, I'm having a hard time making the connection between the
impaired ability of individuals to make it back to the hive because of
the adverse effects of parasites and the impulse of individuals to
commit suicide for the benefit of the colony because of said
parasites. The former seems a much more obvious explanation for why
individuals would not make it back than the latter. Beyond that, I
don't see how one could distinguish between death by suicide and death
as a result physical impairment. Is there something in the studies
that you site that shows more directly how the conclusions are made:
first of all that bees are deliberately leaving to die rather than
just leaving and dieing: and secondly that this would really be of
survival benefit to the colony?

Thanks for the interest, Steve. First off, it is a hypothesis, which
attempts to explain certain observations: that is, the sudden collapse
of bee colonies. I have long thought that it is plausible that the
bees might attempt to rid the colony of pathogens by flying off and
not returning (suicide). Honey bees are unique in their willingness to
sacrifice themselves for the colony as evidenced by their barbed
stingers and other "reckless" behaviours.

However, I sort of kept this theory to myself, because: how would I
prove it? Ms. Kralj and others have been exploring the reasons for
this phenomenon in a much more systematic way and if you following the
progression of their research, initially it was thought that the
non-returning might be caused by some sort of physiological impairment
resulting from infestations. I don't doubt this for one minute. Many
things could feasibly cause shortened lifespan and disorientation in
honey bee foragers. Pesticides, abnormal brood temperatures, and
parasites have been suggested.

Yet, Ms. Kralj is now suggesting that bees may use "suicide" to rid
the colony of pathogens including varroa and nosema. What has caused
this shift, I don't know. Presumably the fact that the abnormal
behaviour it is not limited to bees infested with varroa suggests
something more non-specific. A hypothesis could be formed and verified
in at least two ways. First, as an attempt to explain observed data
that have already been collected. Secondly, one could work from the
other direction and propose an idea such as "what if bees flew off to
rid the colony of parasites, what would that look like?" and then see
if one could support such a hypothesis with observations, such as the
rapid disappearances of older bees, leaving only young bees and the
queen.

Jasna Kralj writes:

> Varroa destructor, an ectoparasitic mite, is one of the most serious pests of the honey bee Apis mellifera causing numerous losses of honey bee colonies worldwide. Life of V. destructor is divided into a reproductive phase in brood and a phoretic phase during which the mite is attached to the adult bee. The mite can leave the colony with workers involved in foraging tasks and may or not return to the colony. That mites do not return to the colony was indicated by recent investigations showing substantially higher infestation of departing workers compared to the infestation of returning workers. The main objective of the study was to provide information whether V. destructor influences flight behavior of foragers and consequently returning frequency of foragers to the colony. The study revealed that infested foragers have extended flights, were impaired in their orientation and return to the colony less frequently. As V. destructor is a relatively recent pest to the western honey bee A. mellifera and specific adaptations cannot be expected, not returning to the colony could be a more general response to diseases and might be a trait which could be enhanced in breeding programs to strengthen the behavioural defence against V. destructor and possibly to other honey bee diseases.

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